1. Field of the Invention
This invention, generally, relates to devices, apparatus and systems for the elimination of unwanted signals from a received, incoming signal and, more particularly, to a circuit for the elimination of unwanted, delayed and attenuated signals received as a part of television signals. In the television field, these unwanted signals are termed "ghosts" and are distracting as well as annoying. The circuit of the present invention is most effective in eliminating such unwanted ghost signals automatically.
The electromagnetic waves in particular ranges of frequencies used in television are easily reflected by buildings or terrain, and it is this characteristic that gives rise to multiple image reception, or "ghosts". These multiple images result in picture degradation and, in frequency modulated (FM) audio reception, causes distortion and loss of fidelity.
2. Description of the Prior Art
As used in television receivers, ghost suppression devices are well known. In such devices, part of a demodulated signal is both delayed and adjusted in polarity and amplitude to offset a ghost signal by adding back into the received television signal in order to "suppress" the ghost signal.
While this may be done manually, an example of an automatic circuit to accomplish such ghost suppression is illustrated in U.S. Pat. No. 4,128,848. Another example of an apparatus to cancel a ghost signal from an incoming video signal is described in U.S. Pat. No. 4,127,874. This apparatus uses a transversal filter to separate and, then, to cancel weighted signals.
In addition, it is well known to use highly directional antennas in order to make the reception of an unwanted, reflected television signal more difficult, thereby solving some of the problems of ghost images. However, in metropolitan areas, this is not very effective because signals reflected from the many high buildings sometimes are about as strong as signals received directly from the television transmitter, and such signals in these areas are received from many different directions.
Several of the more promising techniques used, or suggested, to reduce or to minimize "ghosts" in television picture reception were studied in exhaustive detail, and the results of the study are reviewed in an article entitled, "Performance Evaluations of Selected Automatic Deghosting Systems for Television", by Goyal, Armfield, Geller and Blank and published in the IEEE Transactions on Consumer Electronics, Volume CE-26, February, 1980. The results seem to indicate that, quite clearly, a performance/cost tradeoff will be required in order to obtain a practical deghosting system.
The above-mentioned most excellent work notwithstanding, the present invention provides a ghost signal elimination circuit that is substantially free of unwanted "ghost" signals, and the cost is minimal. There are literally millions of television receivers and FM receivers manufactured annually that need to be free from such unwanted signals. In addition, a ghost signal elimination circuit in accordance with the invention is adaptable to be manufactured separately as an independent article of manufacture for use in retrofitting the many receivers already in use.